In logging while drilling (LWD) geo-steering applications, it is advantageous to detect the presence of a formation anomaly ahead of or around a bit or bottom hole assembly. There are many instances where “Look-Ahead” capability is desired in LWD logging environments. Look-ahead logging is to detect an anomaly at a distance ahead of the drill bit. Some look-ahead examples include predicting an over-pressured zone in advance, or detecting a fault in front of the drill bit in horizontal wells, or profiling a massive salt structure ahead of the drill bit. While currently available techniques are capable of detecting the presence of an anomaly, they are not capable of determining the location of the anomaly with sufficient depth or speed, they are not capable of detecting an anomaly at a sufficient distance ahead of a bit or bottom hole assembly.
In formation evaluation, the depth of investigation of most logging tools, wire line or LWD has been limited to a few feet from the borehole. One such tool is disclosed in U.S. Pat. No. 5,678,643 to Robbins, et al. U.S. Pat. No. 5,678,643 to Robbins, et al. discloses an LWD tool for locating an anomaly. The tool transmits acoustic signals into a wellbore and receives returning acoustic signals including reflections and refractions. Receivers detect the returning acoustic signals and the time between transmission and receipt can be measured. Distances and directions to detected anomalies are determined by a microprocessor that processes the time delay information from the receivers. As set forth above, the depth of investigation facilitated by the tool is limited.
Another technique that provides limited depth of investigation is disclosed in U.S. Pat. No. 6,181,138 to Hagiwara. This technique for locating an anomaly utilizes tilted coil induction tools and frequency domain excitation techniques. In order to achieve a depth of investigation with such a tool, a longer tool size would be required. However, longer tools generally result in poor spatial resolution.
In order to increase depth capabilities, transient electromagnetic (EM) methods have been proposed. One such method for increasing the depth of investigation is proposed in U.S. Pat. No. 5,955,884 to Payton, et al. The tool disclosed in this patent utilizes electric and electromagnetic transmitters to apply electromagnetic energy to a formation at selected frequencies and waveforms that maximize radial depth of penetration into the target formation. In this transient EM method, the current is generally terminated at a transmitter antenna and temporal change of voltage induced in a receiver antenna is monitored. This technique has allowed detection of an anomaly at distances as deep as ten to one hundred meters. However, while Payton discloses a transient EM method enabling detection of an anomaly, it does not provide a technique for detecting anomalies ahead of a drill bit.
Other references, such as PCT application WO/03/019237 also disclose the use of directional resistivity measurements in logging applications. This reference uses the measurements for generating an image of an earth formation after measuring the acoustic velocity of the formation and combining the results. This reference does not disclose a specific method for determining distance and direction to an anomaly.
When logging measurements are used for well placement, detection or identification of anomalies can be critical. Such anomalies may include for example, a fault, a bypassed reservoir, a salt dome, or an adjacent bed or oil-water contact. It would be beneficial to determine both the distance and the direction of the anomaly from the drilling site.
Tri-axial induction logging devices, including wire-line and LWD devices are capable of providing directional resistivity measurements. However, no method has been proposed for utilizing these directional resistivity measurements to identify the direction to an anomaly.
Accordingly, a new solution is needed for determining the distance from a tool to an anomaly. Particularly such a solution is needed for looking ahead of a drill bit. Furthermore, a real time solution having an increased depth of analysis is needed so that the measurements can be immediately useful to equipment operators.